The simplest optical data system stores and recalls data by means of either absorption or reflection of a light beam. A series of data bits can be stored and read by directing a beam of light to a series of data storage locations.
Frequency selective optical data storage (FSDS) systems were first reported by Szabo in U.S. Pat. No. 3,896,420. In FSDS systems, data bits are stored and recalled on the basis of not only their spatial coodinates but also their spectral characteristics. In this method, a storage material, which exhibits an inhomogeneously broadened absorption profile, and which contains many homogeneous absorption channels, is exposed to a highly monochromatic laser beam. At a single spatial location, the laser beam burns a large number of "holes" at discrete absorption frequencies throughout the broad inhomogeneous absorption profile of the storage material. The multi-bits of information stored at each spatial location are recovered by scanning across the material's absorption profile with a highly monochromatic laser, while at the same time monitoring the absorption profile of the storage material to detect the presence or absence of data "holes". The theoretical limit to the number of bits stored at a single location is related to the ratio of the inhomogeneous absorption bandwidth of the material, .DELTA.f.sub.I, to the homogeneous absorption bandwidth of the material, .DELTA.f.sub.H. In some materials, the inhomogeneous width is approximately one million times larger than the homogeneous width. Therefore, theoretically about one million bits of information could be stored in a single region of a few square microns.
The "hole" burning (or frequency-domain) FSDS systems have a number of major drawbacks, one of which is their relative slowness. The maximum speed at which holes can be burned is determined by the relationship .DELTA.t.sub.c .DELTA.f.sub.c .perspectiveto.1, where .DELTA.t.sub.c is the single channel access time and .DELTA.f.sub.c is the single channel spectral width. Therefore, if the storage capacity of the absorbing material in the frequency domain system is increased by making each data channel spectrally narrow, one unavoidably increases the time needed to create and retrieve information from that single data channel. Maximum storage density and maximum access speed are thus incompatible in the frequency domain FSDS systems.
This invention relates to a time domain FSDS method and system, which provides for the storage in one spatial location of a large number of data bits during the time interval required to write one bit in frequency-domain FSDS systems.